graphview: Define link and grid colors in style.css

Previously, these were defined directly in the code,
but defining them in the css helps seperating theming and behaviour
and makes the colors easier to tweak.

src/style.css

@@ -1,14 +1,20 @@
+@define-color audio rgb(50,100,240);
+@define-color video rgb(200,200,0);
+@define-color midi rgb(200,0,50);
+@define-color graphview-link black;
+@define-color graphview-grid @text_view_bg;
+
 .audio {
-    background: rgb(50,100,240);
+    background: @audio;
     color: black;
 }
 
 .video {
-    background: rgb(200,200,0);
+    background: @video;
     color: black;
 }
 
 .midi {
-    background: rgb(200,0,50);
+    background: @midi;
     color: black;
-}
+}